a) Field of the Invention
The present invention relates to a hydrogen plasma down-stream processing technology particularly suitable for removing native oxide or resist on the surface of silicon. In this specification, the term "hydrogen plasma" means plasma of a gas containing hydrogen and includes not only plasma of only hydrogen but also plasma of a mixed gas of hydrogen and another gas.
b) Description of the Related Art
Native oxide on the surface of semiconductor has become an issue of recent semiconductor device manufacturing processes.
The surfaces of most semiconductors and metals are easily oxidized in the air and form native oxide films. A native oxide film formed on the surface of a Si substrate is called an imperfect silicon oxide film and has a thickness of about 2 nm (measured by an ellipsometer). A native oxide film is generally an oxide film formed naturally on the surface of material left in the air. In this specification, the term "native oxide film" is not limited only to such a film, but includes also an imperfect oxide film about 2 nm or less formed on the surface of material subjected to acid treatment or other processes.
A native oxide film on an Si substrate surface is a silicon oxide film having an imperfect crystallinity and having a film quality inferior to a thermally oxidized Si film. As the size of a MOSFET is scaled down, the gate oxide film is becoming as thin as 10 nm or less. For example, in the case when a gate oxide film of 5 nm thick is to be formed, if a poor quality native oxide film of 2 nm thickness is left on the surface of an Si substrate, a resultant gate oxide film will have degraded film characteristics, as a whole.
A wet process using diluted hydrofluoric acid is known as a process for removing a native oxide film on an Si substrate (G. S. Higashi et al., Appl. Phys. Lett., 56, p.656, 1990). A native oxide film on the surface of a Si substrate immersed in a diluted hydrofluoric acid solution dissolves to expose a bare Si surface, and hydrogen atoms terminate dangling bonds at the Si substrate surface.
Although native oxide film removal using diluted hydrofluoric acid forms a stable Si substrate surface on the (1 1 1) plane, it forms a surface with a lower stability on the (1 0 0) plane. This wet process using diluted hydrofluoric acid has some difficulty in directly succeeding to the next dry process, from the viewpoint of a danger that the Si substrate surface may be again oxidized while the substrate is transported to a dry process system.
Attention has been paid recently to processing the surface of semiconductor by using hydrogen atoms. This is because a reducing gas usable in semiconductor device manufacturing is almost only hydrogen. For example, it has bee reported that hydrogen plasma is effective for removing a resist layer used as an ion implantation mask (S. Fujimura, et al., J.J.A.P. 28, p.2130, 1989).
Another known method of removing a native oxide film on a Si substrate is a dry process using hydrogen plasma (A. Kishimoto et al., Jpn. J. Appl. Phys. 29, p.2273, 1990). This technique can essentially and easily be followed by the next dry process because a native oxide film on a Si substrate can be removed by the dry process. However, since a Si substrate is exposed in plasma, the surface thereof may be damaged by collision of high energy particles such as ions and electrons.
It is considered that a native oxide film on the surface of a semiconductor can be removed by action of hydrogen atoms (radicals). A semiconductor substrate placed in hydrogen plasma and radiated with hydrogen radicals has a high possibility of being damaged by high energy particles in the plasma.
One approach to avoiding damages of a semiconductor substrate by plasma is to place the substrate at a down-stream position from the plasma, i.e., to place it down-stream from the plasma where high energy particles are scarce.
However, processing a semiconductor substrate down-stream from the hydrogen plasma poses a new problem that hydrogen atoms (radicals) easily recombine in a region down-stream from the plasma and they become hydrogen molecules. Because a metal surface increases the probability of recombination of hydrogen atoms (radicals), the inner wall of a gas flow path is usually made of fused quartz (silica). If high energy particles in plasma are to be better extinguished, the hydrogen atom concentration should inevitably be lowered greatly such that the process speed becomes slow.
The present inventors have proposed a method of introducing a great number of hydrogen atoms to a semiconductor substrate in the plasma down-stream region by adding water vapor to hydrogen plasma (for example, J. Kikuchi et al. J.J.A.P. 32, p.3120, 1993). Water vapor added to hydrogen plasma prevents hydrogen atoms from being changed to hydrogen molecules, and the hydrogen atoms continue to exist in the plasma down-stream. The reason for this may be a reaction of water vapor with the SiO.sub.2 surface on the inner wall of a processing chamber or the like, which reaction suppresses recombination of hydrogen atoms on the SiO.sub.2 surface.
If the SiO.sub.2 surface is contaminated by some substance attached to the surface, it becomes difficult to introduce a great number of hydrogen atoms to the down-stream region irrespective of addition of water vapor. This difficulty may be ascribed to promotion of recombination of hydrogen atoms (radicals) at the attached contaminant region. Although substance attached to the SiO.sub.2 surface and losing the function of additive water vapor is not certain, it has been confirmed that ethyl alcohol attached to the surface loses the effect of water vapor. For example, if the SiO.sub.2 surface is wiped with a cloth impregnated with ethyl alcohol, hydrogen atoms reduce considerably. Therefore, contaminants losing the function of water vapor may supposedly be carbon, organic material, and other materials.
Contamination may be generated while a substrate is processed, by compositions of the air when a processing chamber is exposed thereto, or other causes.
If the inner wall of a hydrogen plasma down-stream apparatus is contaminated, it is necessary to clean the SiO.sub.2 surface. Cleaning is carried out by dipping the SiO.sub.2 part or surface, for example, in hydrofluoric acid aqueous solution, rinsing it with pure water, and drying it. For such cleaning, SiO.sub.2 components are required to be disassembled from the down-stream apparatus, to be cleaned, and to be reassembled. It is also required to be careful enough not to again contaminate the SiO.sub.2 surface during custody and transport after cleaning.
If contamination is overlooked and a hydrogen plasma down-stream process is performed, a target result of the process cannot be obtained or it takes a long time to complete the process.